Apparatus and method for automated non-destructive inspection of integrated circuit packages

ABSTRACT

An apparatus and method are disclosed for producing acoustical reflected images from selected planes within an integrated circuit package to detect flaws within the package.

This is a divisional of application Ser. No. 07/963,929, filed Oct. 20,1992 which is a continuation of U.S. Ser. No. 07/431,686 filed Nov. 3,1989, abandoned, which is a continuation of U.S. Ser. No. 07/172,043filed Mar. 23, 1988, abandoned.

FIELD OF THE INVENTION

This invention relates to semiconductor devices and more particularly toapparatus and method of not-destructive inspection of integrated circuitpackages.

DISCUSSION OF THE PRIOR ART

Innovations in ultrasonic non-destructive evaluation, or NDE, closelyfollowed advances in electronics technology. As pulse-echo detectionelectronics became more sophisticated, large-scale techniques developedin the sea evolved into laboratory ultrasonic NDE devices. The field ofultrasonic NDE was launched by Floyd Firestone, a physicist at theUniversity of Michigan. In 1942, Firestone received a patent on the"Ultrasonic Reflectoscope" which detected voids or cracks inside ofmanufactured parts by the pulse-echo technique using a contacttransducer. Immersion methods were developed soon thereafter. Thedevelopment of pulse-echo RADAR in 1938 provided the electronicscapability for Firestone's Reflectoscope. Firestone's originalReflectoscope operated in the range of 2-25 MHz which provided awavelength of 0.2-3 mm in steel.

Today's commercial NDE devices are essentially mini-SONAR systems, manyof which can also produce an image, and are remarkably similar inprinciple to the early Reflectoscope.

In typical NDE applications, frequencies in the range of 1-15 MHz areused to penetrate highly attenuating media in search of gross flaws.Penetration is normally the dominant concern in selecting the frequency.Detection and location of a defect are usually more important than highspatial resolution imaging.

A spin-off of the ultrasonic NDE field has been medical ultrasound. Inmedicine, the pulse-echo technique is currently used for imaginginternal organs, for determining fetal viability, and for therapeuticpurposes. Medical ultrasonic imaging uses the same low frequency rangeas typically used for NDE applications in order to penetrate into thehuman body. Although only modest development has occurred in the fieldof NDE since the early Reflectoscope, a great deal of work has been donein improving the pulse-echo apparatus for medical ultrasonic imaging injust the last ten years. Systems using linear arrays or two dimensionalphased arrays are replacing the earlier single transducer designs. Thisgrowth is no doubt stimulated by the considerable commercialopportunities for real-time medical ultrasonic imaging.

The aim of acoustic inspection of plastic-packaged IC's is themicroscopic examination of internal interfaces and defects. In contrastto typical ultrasonic NDE applications, the images produced aremagnified views of sub-surface interfaces. Spatial resolution at theimage depth is as important as penetration, and a compromise is struckbetween spatial resolution and the signal/noise ratio.

Although commercial instrument manufacturers have offered images of thedie-attach layer in ceramic-packaged IC's, very little work has beendone in capitalizing on this capability until quite recently. In 1986,Raytheon completed a report for the Rome Air Development Center ontechniques for imaging die-attach in ceramic-packaged IC's. This reportrecommends pulse-echo acoustic imaging above all other techniques testedincluding x-ray radiography and scanning laser acoustic microscopy(SLAM), due to the contrast and reliability of the pulse-echo image. Thereport made this recommendation in spite of the noted lack of automationavailable for this type of evaluation.

The die-attach layer in ceramic-packaged IC's can be imaged with higherfrequencies than used for plastic package inspection due to the superiorsound transmission in the ceramic material.

Hitachi has very recently release a device called the Scanning AcousticTomograph. This device combines a precession scanning mechanism withtraditional ultrasonic NDE electronics. Only the amplitude informationis detected. Depth information is not recorded and pulse polarityinformation is lost due to signal rectification.

BRIEF SUMMARY OF INVENTION

The present invention is similar to the Panametrics and Hitachipulse-echo systems, but with a novel difference. A specialized signalanalysis system provides automated inspection of packaged integratedcircuits. This system provides three different and important imagessimultaneously. One image is of the reflected amplitude of the firstreflection encountered beneath the surface of the package. This imagelocates features of the internal structure of the integrated circuitpackage. The second image is a y-modulation image or color coded imageof the pulse-echo response time (i.e.: depth of feature). This image isuseful for plotting cracks or inclusions in the plastic above the chipor lead frame; for verifying the depth of the bar, the bar pad, and thelead fingers within the package; and for measuring curvature of the bar.

The third image highlights locations on the first internal interfacesthat produce an inversion of the pulse polarity. This inversion occursat package/void interfaces and therefore conclusively identifiesdelaminations at the bar surface, delaminations on the lead fingersurfaces, package voids, and package cracks.

A single composite image may be generated using, for example, colorcoding to indicate ranges of depth and areas of polarity inversion. Theamplitude image can be corrected for absorption path length differencesusing the depth image.

Alternately, a line scan is made across the sample and an image ofdistance along line scan vs. depth of reflection for all interfacesproducing reflections is made (i.e.: a cross section image of thepackaged part non-destructively). This is performed by on-line capturingof the transient reflection waveform, and mathematically deconvolvingthe reflected pulses to determine amplitude, depth, and polarity of allinternal reflections. These values are used to create two cross sectionimages with axis dimensions of line depth vs. depth. One image is ofamplitude vs. depth, and one is of polarity vs. depth along the linescanned. This permits deconvolving all reflections for a true crosssectional image and not just a first or second subsurface reflectionimage.

Time of data reduction prevents this from being practical for imaging,although in a research environment an entire frame scan producestomographic-like cross sections along any plane through the devicenon-destructively.

The technical advance represented by the invention as well as theobjects thereof will become apparent from the following description of apreferred embodiment of the invention when considered in conjunctionwith the accompanying drawings, and the novel features set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates apparatus for use with the present invention;

FIG. 2 is a 10 Mhz reflection acoustic image of and integrated circuit;

FIGS. 3 and 4 are portions between 17500 and 19500 ns of typicalacoustic reflection spectra form different points on an integratedcircuit package;

FIG. 5 is a deconvolution of the two reflections shown in FIG. 3;

FIG. 6 is an expanded time base view of FIG. 5;

FIG. 7 is a 10 Mhz reflection acoustic micrograph of a second integratedcircuit;

FIG. 8 shows the acoustic reflection spectra for the package/bar andpackage/lead interfaces of the second integrated circuit of FIG. 7;

FIG. 9 is an acoustic spectrum of a crack in the package of FIG. 7; and

FIG. 10 is a block diagram of the detector circuit.

DESCRIPTION OF A PREFERRED EMBODIMENT

The invention uses a pulse-echo principle utilizing an apparatus such asone illustrated in FIG. 1. The system uses the acoustical reflection andis captured in an acoustic microscopy system.

The apparatus illustrated in FIG. 1 includes receptacle 11 having acoupling fluid 12 therein, for example deionized water. A semiconductordevice package 13 encloses the device chip 14. The device, asillustrated may be mounted within the receptacle 11 on its own leadwires 21.

A pulse generator 15 generates and sends a pulse, for example 10 Mhz, totransducer 16. The pulse is transmitted through and focused by lens rod17 to a sound beam as indicated by 18. This beam 18 is scanned over thesurface of device 14. The echo of each pulse is reflected back to lensrod 17 through coupling fluid 12. The echo signal is transmitted fromtransducer 16 to detector circuit 19, and the three detected parametersare stored in frame store 20. The frame store memory is loadedsynchronously with the transducer (or sample) scan index.

It is necessary to reduce the sound frequency to 30 MHz (30 Mhz is usedin the prior art) before significant penetration is possible.

The shape and structure of a plastic-packaged IC are ideal for imagingwith a single lens that is scanned in a two-dimensional raster over theIC. The piezoelectric transducer is extremely sensitive and the singleelement acoustic lens is designed, and required, to perform well only onaxis. The path length through the highly attenuating package mixture isalways minimized. The pulses can be transmitted and received by a singletransducer that is mechanically scanned, or with an array of transducerelements that are electronically scanned.

The absence of an irregular surface morphology on the package topsurface maximizes the sensitivity to fine detail in images formed fromsub-surface interface reflections. The transducer is scanned in a planethat is parallel to the planar, layered internal components in the ICpackage. This means that all of the internal structures lie close to theideal focal plane. And finally, the deionized water bath and the levelof insonification are considered acceptable for a non-destructive test.

For mathematical convenience it is assumed that we are dealing with thenormal incidence of plane sound waves on plane interfaces, and that thematerials are rigidly bonded, ideal elastic solids. Therefore, theacoustic pressure and particle velocity are considered to be continuousacross an interface. The effects of well bonded, thin layers atinterfaces, particularly at the bar surface, are ignored at this timebecause the thicknesses of these layers are much less than one acousticwavelength in materials in the frequency range used.

Under these assumptions, there are three primary sources of informationin the acoustic pulse-echo spectra recovered from a plastic-packaged IC.These are the depth of the reflection, its polarity relative to theincident pulse, and the amplitude of the reflection.

Measurements of the depth of a reflecting sub-surface interface areperhaps the most straightforward analyses that can be performed onacoustic pulse-echo spectra. This is a unique characteristic of thepulse-echo technique. A transmission technique provides informationabout total transmitted energy but not about the depths of interfaces ina practical method.

The depth of an interface is determined by measuring the delay betweenthe arrival of the echo from the front surface of the package and theecho from the sub-surface interface. This time is the "round trip" timedifference, or twice the time required to penetrate from the frontsurface to the sub-surface interface. Knowing the speed of sound in thetraversed layer(s) enables one to easily calculate the depth of thesub-surface interface. In acoustic inspection of the integrated circuit,of most importance is the first sub-surface interface. In some cases thesecond sub-surface interface is interesting. In this case the secondlayer is relatively thin. In both cases, the thicknesses of thetraversed layers are constant across the image plane.

The analysis of acoustic vibration is based on simple harmonicoscillation and is quite similar to classical electromagnetic radiationtheory. In fact, the form of the acoustic reflectivity of an interfaceis similar to that for the optical reflectivity of an interface inoptically transparent solids.

The optical reflectivity is based on the indices of refraction of thetwo layers. The acoustic reflectivity is based on the acousticimpedances of the two materials.

The characteristic acoustic impedance of layer n is given by:

    Z.sub.n =P.sub.n *V.sub.n                                  (1)

where P_(n) is the density of layer n and V is the acoustic phase speedin layer n. The pressure reflection coefficient, R, is simply the ratioof the reflected pressure amplitude to the incident pressure amplitude.The form of the pressure reflection coefficient, R, follows directlyfrom the assumptions of continuity of pressure and particle velocityacross the interface: ##EQU1## The subscripts 1 and 2 refer to thematerial on the incident and on the transmitted side of the interface,respectively. When Z₂ >Z₁, the reflected pulse has the same polarity asthe incident pulse. However, when Z₂ <Z₁, the reflected pulse isinverted, or has opposite polarity, with respect to the incident pulse.This polarity inversion principle will be demonstrated to be quiteuseful for detecting voids and delaminations in plastic-packaged IC's.

Since the reflected and incident pressure waves sum to equal thetransmitted pressure wave at the interface, the pressure transmissioncoefficient is given by: ##EQU2##

Since T is always positive, the transmitted pulse is always of the samepolarity as the incident pulse.

It can be seen from Equation 2 that the amplitude of the echo pulse is afunction of the difference in the characteristic acoustic impedances ofthe materials meeting at the interface. For an ideal interface betweenmedia of identical characteristic acoustic impedance, there is noreflection. And, at the interface between media of very differentcharacteristic acoustic impedances, there will be a strong reflection.The amplitude of the echo pulse in strongly attenuating media is alsoaffected by the depth at which the reflection occurs.

FIG. 2 is a 10 MHz reflection acoustic image of a 68-pin PLCC to beidentified as Package A. This packaged IC has undergone one week of85/85 (85° C., 85% tel. humidity) followed by VPR (vapor-phase reflowsoldering). The expansion of trapped moisture during VPR has resulted indelamination at the package/bar interface initiating at the bar corners.The delaminated bar surface areas appear brighter in FIG. 2.

This type of failure seriously affects the reliability of the gold wirebonding and the dissipation of heat from the bar.

FIGS. 3 and 4 are the portions between 17500 and 19500 ns of typicalacoustic reflection spectra from different points on Package A. FIG. 3shows the echo spectra of the reflections from the package/bar interfaceat the center of the bar. The package/lead interface in Package Aoverlaid for comparison. FIG. 4 shows the echo spectra for thepackage/bar interface at locations of good adhesion and of delaminationoverlaid for comparison.

The reflected pulse at 17500 to 18000 ns that is essentially common toall these spectra is from the package surface. Since the acousticimpedance of water is less than that of the package material, thepackage surface reflection must be of the same polarity as that of theincident pulse.

The acoustic impedance of the package material is less than that ofsilicon or of the metal lead frame material, so both the package/leadand the package/bar interface reflections in FIG. 3 are of the samepolarity as the package surface reflection, and therefore of the samepolarity as the pulse incident on the package surface.

The package/bar reflection in FIG. 3 is taken over the center of the barin Package A where the package remains adhered to the bar surface. Thisis the same spectrum as that overlaid with the spectrum of a package/bardelamination shown in FIG. 4. Note that the package/bar interfacereflection in FIG. 3 is actually two reflections superposed. FIG. 5 is adeconvolution of these two reflections shown on an expanded time base.The second reflection is delayed due to the penetration throughapproximately 450 μm of silicon twice and is from the bar/die-attachinterface. A unique lobe on this second, or die-attach, pulse is used toimage voiding in the die-attach layer.

The characteristic acoustic impedance of a void, or of package materialthat has penetrated under the bar into a die-attach void, will also beless than that of silicon. The characteristic acoustic impedance of avoid will is assumed to be that of air. Therefore, delamination at thebar/die-attach interface will not be distinguishable by localized phaseinversion.

FIG. 6 is an expanded time base view of FIG. 4 showing two typicalreflections from the package/bar interface in Package A. Note that thereflection from the delaminated interface is of opposite polarityrelative to that from the location showing adhesion.

This is due to the fact that the void has a lesser characteristicacoustic impedance than does package material. And, as noted earlier,silicon has a greater characteristic acoustic impedance than doespackage material. Therefore, at the package/bar interface, thereflection from the location showing good adhesion has the same polarityas the pulse incident on the package surface, while the reflection froma delaminated location will have opposite polarity. Also, the reflectionfrom the delaminated location is a single pulse since the die-attachinterface at this location is in the shadow of the delamination,acoustically speaking.

Since the acoustic impedance mismatch is greater at the delaminatedlocation, under ideal focusing conditions, this reflection has a greateramplitude than that from the location showing adhesion. Therefore,locations of delamination at the package/bar interface may bedistinguished by a locally greater reflected amplitude, under idealconditions, and are always distinguished by a locally inverted pulsepolarity. However, delaminations at the package/lead interface are notconclusively distinguishable by reflected pulse amplitude, and pulsepolarity detection is critical for imaging these areas.

FIG. 7 is a 10 MHz reflection acoustic micrograph of another 68-pin PLCCidentified as Package B. Package B was exposed to one week of 85/85followed by the solder dip process. The entire package/bar interface isdelaminated, as revealed by pulse polarity analysis.

FIG. 8 shows the acoustic reflection spectra for the package/bar andpackage/lead interfaces. Note that the package/bar reflection hasinverted polarity relative to the pulse incident on the package surface.Also, note that the bar pad is significantly higher in the packagecompared to Package A.

There are two distinct acoustic shadows in FIG. 7 in the upper leftcorner of Package B. A time window enclosing the package/bar andpackage/lead interfaces was used to form the image in FIG. 7. Theacoustic shadows are formed by cracks in the package above thepackage/bar interface. Reflections from these cracks are therefore notincluded in the image in FIG. 7. Since these cracks exhibit totalreflection, all interfaces below them are acoustically shadowed. So thecracks are revealed in FIG. 7 by the acoustic shadows they create. Thepresent invention will image such cracks and identify them by theirdepth and profile.

An acoustic spectrum at the location of one of these cracks is shown inFIG. 9. The reflection from the crack partially interferes with thepackage surface reflection and no later reflections are detected.

FIG. 10 is a block diagram of the detector circuit used in the presentinvention. The pulse-echo signal is gated in time using the main trigger(T₀), and a time delay, to indicate only the package surface reflection.

Threshold trigger circuit 20 detects the integrated circuit packagesurface reflection and generates a timing trigger T₁. The pulse echosignal is gated again (in gate 21) using T₁, using e time delay toindicate only the reflections from inside the integrated circuitpackage. The delayed pulse-echo signal present at point P₄ isillustrated in FIG. 4.

A gate pulse G₁ is generated in gate 21 to indicate the occurrence andduration of the gate spectrum of the reflections inside the package. Thespectrum of reflections from inside the integrated circuit package isoutput from gate 21 and is present at point P₆. The spectrum ofreflections present at point P₆ is illustrated in FIG. 6.

The spectrum of reflections form inside the integrated circuit packageis passed into the bipolar peak detector 24 which generates a timingtrigger T₂ when a peak above a set threshold is detected. The bipolarpeak detector 24 is enabled by the gate pulse G₁ from gate circuit 21.

The bipolar peak detector circuit 24 also generates an indication of thepolarity of the first internal reflection by sensing the polarity of thefirst major positive lobe in either the inverted or non-inverted pulse.An automatic gain control amplifier and threshold trigger (in block 24)or a constant fraction discriminator, or similar circuit is used as alobe detector. This polarity indication, for example: a D.C. leveloutput of +5 V for inverted and 0 V for normal, is used to form thepolarity image.

The spectrum of reflections from inside the integrated circuit is alsorectified and smoothed (block 25) to provide a pulse that is alwayspositive for amplitude detection.

A sample-and-hold circuit 27 is used to capture a representativeamplitude, from the rectify and smooth circuit 25, for the firstinternal reflection. Trigger T₂ is used to hold the amplitude level andthe gate pulse G₁ from the second signal gate 21 is used to enable thesample-and-hold amplifier. This representative amplitude is used to formthe amplitude image. There is an optional delay circuit 26 which may beused to delay trigger T₂ before it is applied to the sample-and-holdcircuit 27.

Trigger T₁ , from delay-signal gate 20 is delayed by calibration delay22 to compensate for the artificial delay added to trigger T₂ bydelay-signal gate 21 and the bipolar peak trigger circuit 24, and thedelay of trigger T₁ is calibrated.

The delayed trigger T₁ and trigger T₂ are input into a time-to-amplitudeconverter 23. A DC level is generated which corresponds to the delaybetween the triggers, and therefore corresponds to the depth of thefirst internal interface. The output of the Time-to-Amplitude circuit 23is used to form the depth image.

Each of the three images, Amplitude, Depth, and Polarity is stored inseparate frame store memory (not illustrated), and the location of eachimage within each frame store is scanned synchronously with the locationof the scanned transducer.

What is claimed:
 1. A detector circuit for receiving pulse-echo signalsreflected from a scanned integrated circuit package and outputtingsignals representative of three images, comprising: a delay gate forgenerating a first timing signal and a signal representative ofpulse-echo signals of reflection from inside of the integrated circuitpackage; a gate circuit to generate a gate signal and a spectrum ofreflection from inside of the integrated circuit package; a bipolar peakdetector for generating a signal indicative of the polarity of a firstinternal pulse-echo from inside of the integrated circuit package andfor generating a second timing signal; a sample-and-hold circuit forcapturing a representative amplitude of a pulse-echo; and atime-to-amplitude for receiving said first and second timing signals toproduce a signal indicative of depth of a pulse-echo signal.
 2. Thedetector circuit according to claim 1, including a circuit to rectifyand smooth the spectrum of reflection from inside of the integratedcircuit package to provide a pulse that is always positive for amplitudedetection.
 3. The detector circuit according to claim 1, wherein saidfirst timing signals is delayed and calibrated prior to the time it isreceived by said time-to-amplitude circuit.